Practical machinery safety

1 Introduction to the machinery safety workshop .................................................... 1 1.1 Scope and objectives ........................................................................... ...2 1.2 Machinery and controls ........................................................................ ...2 1.3 Distinction between machinery and process safety control systems..... ...7 1.4 International standards and practices ................................................... ...8 1.5 Introduction to hazards and risks.......................................................... .10 1.6 Risk reduction....................................................................................... .11 1.7 The Alarp principle for tolerable risk ..................................................... .12 1.8 Development example for a machinery safety system.......................... .14 1.9 The engineering tasks .......................................................................... .19 1.10 Benefits of the systematic approach ..................................................... .22 1.11 Conclusions.......................................................................................... .23 2 Guide to regulations and standards ................................................................... 24 2.1 Purpose and objectives ........................................................................ .24 2.2 History and overview of European Directives and Standards ............... .25 2.3 The European Machinery Directive....................................................... .33 2.4 Conformity procedures ......................................................................... .39 2.5 Other ‘New Approach Directives’ .......................................................... .44 2.6 User side directives: workplace health and safety legislation ............... .46 2.7 Some machinery safety standards........................................................ .49 2.8 Regulations and standards in the USA................................................. .52 2.9 Conclusions.......................................................................................... .55 References ........................................................................................... .55 3 Risk assessment and risk reduction ................................................................... 56 3.1 Purpose and objectives ........................................................................ .56 3.2 Introduction to risk assessment ............................................................ .56 3.3 Procedure for risk assessment ............................................................. .57 3.4 Hazard study methods.......................................................................... .66 3.5 Risk estimation ..................................................................................... .71 3.6 Risk reduction principles....................................................................... .79 3.7 Outcomes of the risk assessment......................................................... .85 3.8 Documentation methods for the risk assessment ................................. .90 3.9 Conclusions.......................................................................................... .91 References ........................................................................................... .91 vi Contents 4 Design procedures for safety controls................................................................ 92 4.1 Introduction to design techniques ......................................................... 92 4.2 Review of design standard EN 954-1.................................................... 93 4.3 Procedure for the design of safety controls based on EN 954 .............. 94 4.4 Design considerations...........................................................................97 4.5 Safety categories ................................................................................ 104 4.6 Conclusions ........................................................................................ 110 References ......................................................................................... 111 5 Emergency-stop monitoring and the safety relay ............................................. 112 5.1 Introduction......................................................................................... 112 5.2 Definitions and implications of stop functions...................................... 112 5.3 Safety relay terminology ..................................................................... 114 5.4 How does an E-stop safety relay work? .............................................. 116 5.5 Practical safety relays ......................................................................... 117 5.6 Certification......................................................................................... 125 5.7 Functional overview of monitoring relays ............................................ 125 5.8 Electronic and programmable E-stop monitors ................................... 127 5.9 Using monitoring safety relays for guards (safety gate monitors)........ 128 5.10 Review of other monitoring relay functions ......................................... 128 5.11 Conclusions ........................................................................................ 130 References ......................................................................................... 131 6 Sensors and devices for machinery protection ............................................... 132 6.1 Contents summary.............................................................................. 132 6.2 Purpose and objectives....................................................................... 132 6.3 Review of guards ................................................................................ 138 6.4 Sensing devices for guards................................................................. 144 6.5 Mechanical trapped key interlocking ................................................... 152 6.6 Presence sensing devices .................................................................. 154 6.7 Control devices for safety.................................................................... 163 6.8 Safety networks and sensors .............................................................. 166 6.9 Conclusions ........................................................................................ 168 7 Application guidelines for protection devices ................................................... 169 7.1 Introduction......................................................................................... 169 7.2 Choosing protection methods ............................................................. 170 7.3 Guarding devices ................................................................................ 171 7.4 Point of operation devices................................................................... 173 7.5 Application guidance notes for light curtains ....................................... 180 7.6 Conclusions ........................................................................................ 188 8 Programmable systems for safety controls ...................................................... 190 8.1 Introduction......................................................................................... 190 8.2 Benefits and disadvantages of safety PLCs........................................ 195 8.3 Characteristics of safety PLCs............................................................ 201 8.4 Application software............................................................................ 214 Contents vii 8.5 Safe networking....................................................................................215 8.6 Classification and certification of safety PLCs ......................................218 8.7 Summary..............................................................................................219 References...........................................................................................219 9 Introduction to standards for programmable systems ........................................220 9.1 Introduction...........................................................................................220 9.2 Objectives.............................................................................................220 9.3 Outline of IEC 61508 ............................................................................221 9.4 Concept of SILs....................................................................................226 9.5 How can we determine the required SIL for a safety function?.............228 9.6 Some implications of IEC 61508 for machinery systems ......................230 9.7 Summary..............................................................................................232 9.8 Conclusion............................................................................................232 References...........................................................................................233 Appendix: Notes on the method for the determination of SILs for a machinery safety application........................................................234 Appendix A: References and sources of information on machinery safety ...................240 Appendix B: Glossary ...................................................................................................243 Appendix C: Notes on tolerable risk..............................................................................248 Appendix D: Notes on PUWER ....................................................................................252 Appendix E: Guide to fault tree analysis .......................................................................257